After printing on material webs intended for printing in a rotary printing press, the individual web strands are brought together at a collection roller into one or more strands to be bound or stitched (connected). One strand to be bound runs over a folding hopper into a folder, in which the strand to be bound is cut at right angles to the direction of delivery by a cutting cylinder. The position of this cut relative to the printing style on the printed web is an important variable in the production of a printed product. The measurable length of the printing style relative to the position of the cutting cylinder is called the cutting position. The cutting position is usually set at the beginning of a production, but it must be continually adjusted during the running production. The cutting position is changed by a so-called register mechanism, which is arranged before the web strands are brought together. The register mechanism typically consists of main and secondary registers. A register for the strand to be bound, which is arranged after the individual web strands have been brought together, may additionally be used as well. The cutting position for a web strand is set by a change in the length of the path of the web. To control the register means, measured values for the cutting positions of the web strands are recorded and evaluated in terms of control engineering in order to control the register means.
It has been known that measured values for the cutting positions of the web strands can be recorded by means of optical sensors. The crop mark is usually set by the printer correctly at the control station, after which the control is switched on and the cutting position is determined at the sensor during the first revolution and stored. The control subsequently checks during each revolution whether the cutting position has shifted in relation to the position stored during the first revolution. If it has, the control calculates a correction signal, which again displaces the cutting position at the sensor in the direction of the desired position being stored.
The optical sensors used to determine measured values for the cutting positions of the web strands are arranged in an area located in front of the area in which the individual web strands are brought together. The printed webs usually still travel over a considerable distance from this measuring point to the cutting cylinder. This in turn implies a considerable potential of disturbance in the cutting positions of every individual web strand. The path of the web from the site of the measurement to the cutting cylinder is, in general, so long that cutting position deviations in the millimeter range will result in the usual printing operation. For example, a change in the stretching of the web during production between the site of the measurement and the cutting cylinder in the folder leads to a change in the cutting position because the cutting position at the sensor is maintained by the controller at a constant value and the number of images located between the sensor and the cutting cylinder will no longer be correct now. The stretching of the web may change due to a difference in ink application; in addition, it also depends on the humidity of the ambient air. Another problem is represented by control units, which, when viewed in the direction of travel of the paper web, are arranged after the sensor, e.g., a register for the strand to be bound. Adjustments with such control units do not become visible in the measured signal, but they do cause a change in the cutting position. However, what disturbs the cutting position in practice is above all the change from one speed of rotation to another. This happens, e.g., at the time of the start-up of a press, or after a paper break. The farther away the sensors are located from the cutting cylinder, the more noticeable are such effects as deviations in the cutting position.
A process in which sensors can be arranged at a short distance in front of the cutting cylinder has been known from DE 195 06 774 A1. The measurement of the cutting positions of the web strands is thus performed in the strand to be bound, i.e., after the individual web strands have been brought together. A reference mark consisting of a magnetizable special ink is applied to every individual printed web during the printing operation. These reference marks are themselves magnetized during running through an external magnetic field and can be subsequently recognized by a magnet sensor. The advantage of this method is that measurement values can be determined for the cutting positions of the web strands even in the web strands that are covered by other web strands after they have been brought together and are thus invisible to optical sensors. The magnetic fields of the reference marks must be only strong enough to penetrate the superjacent web strands. The magnet sensors may be arranged, laterally offset, directly in front of the cutting cylinder, and thus they recognize any change in the cutting position. A marking means is necessary for applying reference marks, and a magnetizing means is necessary for the reference marks applied. Space for installing a plurality of sensors must be present at a short distance in front of the cutting cylinder.